built-in-references/Kubernetes/allowed-host-paths/template.yaml

apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8sazurehostfilesystem
spec:
  crd:
    spec:
      names:
        kind: K8sAzureHostFilesystem
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          properties:
            allowedHostPaths:
              type: array
              items:
                type: object
                properties:
                  readOnly:
                    type: boolean
                  pathPrefix:
                    type: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sazurehostfilesystem

        violation[{"msg": msg, "details": {}}] {
            volume := input_hostpath_volumes[_]
            allowedPaths := get_allowed_paths(input)
            input_hostpath_violation(allowedPaths, volume)
            msg := sprintf("HostPath volume %v is not allowed, pod: %v. Allowed path: %v", [volume, input.review.object.metadata.name, allowedPaths])
        }

        input_hostpath_violation(allowedPaths, volume) {
            # An empty list means all host paths are blocked
            allowedPaths == []
        }
        input_hostpath_violation(allowedPaths, volume) {
            not input_hostpath_allowed(allowedPaths, volume)
        }

        get_allowed_paths(arg) = out {
            not arg.parameters
            out = []
        }
        get_allowed_paths(arg) = out {
            not arg.parameters.allowedHostPaths
            out = []
        }
        get_allowed_paths(arg) = out {
            out = arg.parameters.allowedHostPaths
        }

        input_hostpath_allowed(allowedPaths, volume) {
            allowedHostPath := allowedPaths[_]
            path_matches(allowedHostPath.pathPrefix, volume.hostPath.path)
            not allowedHostPath.readOnly == true
        }

        input_hostpath_allowed(allowedPaths, volume) {
            allowedHostPath := allowedPaths[_]
            path_matches(allowedHostPath.pathPrefix, volume.hostPath.path)
            allowedHostPath.readOnly
            not writeable_input_volume_mounts(volume.name)
        }

        writeable_input_volume_mounts(volume_name) {
            container := input_containers[_]
            mount := container.volumeMounts[_]
            mount.name == volume_name
            not mount.readOnly
        }

        # This allows "/foo", "/foo/", "/foo/bar" etc., but
        # disallows "/fool", "/etc/foo" etc.
        path_matches(prefix, path) {
            a := split(trim(prefix, "/"), "/")
            b := split(trim(path, "/"), "/")
            prefix_matches(a, b)
        }
        prefix_matches(a, b) {
            count(a) <= count(b)
            not any_not_equal_upto(a, b, count(a))
        }

        any_not_equal_upto(a, b, n) {
            a[i] != b[i]
            i < n
        }

        input_hostpath_volumes[v] {
            v := input.review.object.spec.volumes[_]
            has_field(v, "hostPath")
        }

        # has_field returns whether an object has a field
        has_field(object, field) = true {
            object[field]
        }
        input_containers[c] {
            c := input.review.object.spec.containers[_]
        }

        input_containers[c] {
            c := input.review.object.spec.initContainers[_]
        }